Froth flotation is one of the most widely used technologies for mineral extraction/separation from mined mineral-bearing ores, such as base metals, ferrous metals, industrial minerals, coal and oil sands (tar sands) prior to further refinement (processing). As early as in 1869, William Haynes first patented a flotation process for separating sulfide minerals from gangue (waste material) with oil. In 1905, the froth flotation process was simultaneously invented by A. H. Higgins in England and by G. A. Chapman in Australia with the use of mostly naturally occurring chemicals, such as fatty acids and oils as flotation aids. Since then, efforts have been made to improve this process technology in the areas of flowsheets, flotation aids and equipment.
Froth flotation is a selective process or separating different finely divided materials, especially valuable minerals, from waste or unwanted materials. The process generally consists of agitating a mixture of the materials with water and additive chemicals (slurry). This slurry is then introduced to a flotation cell which is aerated, creating bubbles as separating media. Hydrophobic particles stick to the bubbles and the hydrophilic particles remain in the aqueous phase. So flotation involves the hydrophobic material particles (e.g., minerals) being carried by air bubbles to the slurry surface, forming a foam layer (froth layer). Accordingly, the process involves a partitioning between the “floated” particles and the non-floated particles which remain in the slurry. The froth (containing the floated material) is separated as a concentrate that is further processed (refined). In normal flotation, the slurry left in the flotation cell is called tailings and typically needs to be further treated in a solid-liquid separation step to reuse the water and consolidate the solids to be discharged as waste “gangues” (in minerals extraction) or “ash” (in coal processing). Since most of the valuable minerals and waste materials are naturally hydrophilic, chemical reagents are typically added which enhance or increase the hydrophobicity of the valuable mineral. Alternatively, the reagents may be added to increase the hydrophobicity of the waste material (e.g., gangue) in reverse flotation. With the aid of such selective additives (flotation aids) selective flotation separation between, for instance, valuable mineral and gangue, is possible. The differential wetting (hydrophobic or hydrophilic) of valuable particles relative to unwanted particles needs to be achieved to a sufficient extent to allow differential attachment to bubbles. Generally, the flotation process is called normal flotation if the valuable particles are floated; otherwise, it is named reverse flotation with the unwanted particles (e.g., gangue) being floated. Flotation aids are mainly classified into the following three following categories: dispersants/depressants, collectors, and frothers.
Dispersant/depressant: The purpose of using dispersant is to eliminate the heterocoagulation between two different particles (for instance, mineral particles) or liquid droplets via increasing the repulsive interactions between them (electrical double layer repulsive forces, hydration force, steric force). The most commonly used dispersants in mineral flotation include water glass, sodium hexametaphosphate (Calgon), dextrin, sodium fluorosilicate, CMC (carboxymethyl cellulose), gums, polysaccharides, tannic acid, lignosulfonates, and some small molecular weight polymers etc. When added into the slurry, the dispersant adsorbs onto the particles' surfaces and leads to a very high repulsive potential energy barrier to prevent the particles attaching to each other, making the ore slurry a well dispersed suspension of individual particles. At the same time, the dispersant also plays the role as a depressant, which is supposed to make the gangue minerals more hydrophilic and difficult to attach to the air bubbles, because the dispersant/depressant reagent adsorption layers on the solid surfaces are usually hydrophilic. For example, both natural and synthetic polymers have been used to depress talc in known flotation processed. It is believed that the polymer's mode of action involves the polymer adsorbing onto the gangue particle surface via one or more of several binding mechanisms which makes the gangue particles more hydrophilic due to the existence of polymer functional groups thus preventing bubble-particle attachment.
Collector: There is almost no driving force for material particle-air bubble attachment in water if the material particles are hydrophilic. (Hydrophilic in this sense means that there is a low contact angle of the air-water-solid interface, where contact angle is measured from inside the water. Hydrophobic surfaces are on the other hand those where the contact angle between the air-water-solid interface as measured from inside the water is high.) The efficiency of flotation separation is strongly dependent on the degree of hydrophobicity of the particles to be floated. The hydrophobicity of particles can be enhanced by the addition of a collector into the slurry which adsorbs onto the particles surface. The degree of particle hydrophobicity depends on the type and amount of additive adsorbed onto the solid particle surface, and is known to increase the material flotation recovery. For mineral flotation, some organic molecules are used as collectors depending on the ore types. For example, alkyl xanthates are commonly used as collectors for sulphide mineral flotation; for the flotation of oxide ores (hematite, silica, etc.) there are more reagent choices like dodecanoic (lauric) acid, dodecanoic hydroxamate, sodium dodecyl sulphate (SDS), sodium dodecyl benzyl sulphonate (SDBS) and dodecylamine salts. For coal, oils like diesel and kerosene are used as collectors.
Frother: The primary purpose of using frother in flotation is to create air bubbles in suitable numbers (preferably many) and sizes (preferably small and uniform) to facilitate the mineral particle-bubble attachment and consequently the recovery of minerals. Frothers also help stabilize the froth (foam) at the top of the flotation cell so that it may be separated from the aqueous suspension often called the pulp. Frothers can essentially be divided into four groups. The first is aromatic alcohol types, such as aliphatic-cresol and 2,3-xylenol. The second is alkoxy types such as triethoxy butane (TEB). The third is aliphatic alcohol types such as 2-ethyl hexanol, diacetone and methyl isobutyl carbinol (MIBC), which is the most commonly used single frother today as it is relatively inexpensive and has good performance with different ores. The fourth is synthetic frothers consisting of PEO (polyethylene oxide), PPO (polypropylene oxide) and PBO (polybutylene oxide) types.
Flocculants are a further reagent sometimes used in conjunction with flotation aids in flotation processes. A flocculant induces attraction between particles so that they aggregate into larger more massive aggregates called flocs. Typically the use of flocculants is aggregation in solid/liquid separation (e.g., in the treatment of raw water in order to produce potable water). Both natural and synthetic polymers can be used as flocculants, such as starch and polyacrylamide based polymers that are very effective to destabilize the fine particle suspension.
Flocculants may be used in a hydrophilic flocculation-flotation process. Such a process is used in the case of hematite/silica separation. The fine hematite particles are first selectively flocculated by starch making them hydrophilic and increase their settling rate and then the silica particles are activated by a silica collector and floated through reverse flotation.
Another use of water soluble high molecular weight polymeric flocculants is to aid solid-liquid separation in tailings treatment (or product consolidation) after mineral separations such as flotation. The action mechanism of conventional polymeric flocculants is believed to be that the polymer induces inter-particle attractive forces by adsorbing onto and bridging between multiple particles. This is thought to lead to an increase in the effective particle diameter and mass, in turn leading to an increase in the rate of solid settling and more efficient solid-liquid separation. Unfortunately, conventional polymeric flocculants produce sediments and filter cakes which do not consolidate well. They typically contain high amounts of residual liquid because of the open, strong floc structure that results from bridging flocculation.
In conventional mineral processing applications, the flotation aids mentioned above are usually added one at a time in such a sequence such as dispersant/depressant, collector, and frother. Therefore the characteristics of the conventional approach to mineral flotation and tailings treatment are not optimized. The following brief description illustrates this point.
In an overall mineral flotation process, typically as many as three or four types of reagents are used, which results in high reagent purchase cost, high production cost for preparing and adding these reagents, deteriorated recycle water chemistry (harmful ions or components remain in the water), adverse effect on the following tailings treatment due to the high dose addition of dispersants, and adverse effect on the environment if the aid additives used in flotation are discharged as waste. For example, there has been environmental concern with regard to the low flash point temperature and high vaporization rate of MIBC that produces an unpleasant odor in warmer climates.
Second, during the tailings treatment (solid-liquid separation) that includes solids aggregation/settling and sediment consolidation, the polymeric flocculants that are used to help the fine solid particle settling down quickly are not optimized for sediment consolidation. Although conventional poly acrylamide (PAM) based polymers work well in helping solids settling, a large amount of water is actually trapped in the flocs formed by conventional polymeric flocculants. This has an adverse effect on the water release during the stage of sediment consolidation. The polymer action mechanism for bridging flocculation is adsorption of large MW polymer onto multiple particle surfaces which results in the formation of large and loose flocs. The strong attraction that results, accounts for this type of polymer's performance during dewatering. Efficient solids dewatering is urgently required to meet the challenges of water supplies and environment protection.
Finally and most importantly, conventional flotation and tailings treatment processes are considered in isolation of each other when considering the selection and application of reagents. A typical example is the application of a dispersant in mineral flotation and flocculant addition for tailings treatment. The strong dispersion of fine particles in the flotation brings difficulty for fines flocculation in the tailings treatment, because the strong inter-particle repulsive forces induced by the dispersant during flotation needs to be changed to attractive forces during solid-liquid separation. This usually means that additional flocculant is required to induce the polymer bridging effect.
Based on the above one can appreciate the need to develop more efficient flotation extraction and processing methodologies as well as develop more versatile flotation aids.